What is the ruling of Islamic law regarding the marriage of a young man accused of moral issues and criminal records to a girl who insists on marrying him despite her family's rejection, and the young man's mother claims to have suckled the girl, while the girl's mother claims to have suckled the young man, which raises the suspicion of prohibitive suckling between them?

What is prohibited by fosterage is prohibited by lineage. If the young man was nursed by your mother, he is your brother through fosterage, and he is not permitted to marry any of you [sisters]. Similarly, if your sister was nursed by his mother, she is his sister through fosterage, and she is not permitted to marry him, provided that the nursing occurred within the first two years of life, with five full, satisfying breastfeeds. Therefore, neither her father nor any of her guardians may contract her marriage to this young man. If she marries him without a guardian, her marriage is invalid from the perspective of the guardian and the prohibited degrees of relationship. Furthermore, this young man is not a suitable husband based on his conduct. You should advise your sister, protect her, and hasten her marriage [to someone else].